Differences in backscattering strength determined at 120 and 38 kHz for three species of Antarctic macroplankton

The ability to acoustically separate zooplankton species is an important requirement for ecological studies and to improve biomass estimates. In order to distinguish between Euphausia superba and other swarm-forming macroplankters we used a dual frequency echo-sounder (120 and 38 kHz) and echo-integrator during a series of Longhurst Hardy Plankton Recorder (LHPR) hauls near South Georgia. We compared the acoustic parameter Mean Volume Backscattering Strength (MVBS) according to the equation: DELTAMVBS (dB) = MVBS 120 kHz - MVBS 38 kHz. Mean values of DELTAMVBS for E. superba, Themisto gaudichaudii and E. frigida were 4.6, 9.7 and 15.6 dB, respectively, and were significantly different, allowing the 3 species to be separated acoustically.